Buoyancy element, riser assembly including a buoyancy element and a method of supporting a riser

ABSTRACT

A buoyancy element, riser assembly including a buoyancy element and a method of supporting a riser are disclosed. The buoyancy element includes a body of at least partly flexible structure having a volume that is adjustable between an expanded and contracted position, and an entry port for fluid to be supplied to and/or removed from the body to thereby adjust the volume of the body.

The present invention relates to a buoyancy element, riser assembly including a buoyancy element, and a method of supporting a riser. In particular, but not exclusively, the present invention relates to a riser assembly suitable for use in the oil and gas industry.

Traditionally flexible pipe is utilised to transport production fluids, such as oil and/or gas and/or water, from one location to another. Flexible pipe is particularly useful in connecting a sub-sea location (which may be deep underwater) to a sea level location. Flexible pipe is generally formed as an assembly of a flexible pipe body and one or more end fittings. The pipe body is typically formed as a combination of layered materials that form a pressure-containing conduit. The pipe structure allows large deflections without causing bending stresses that impair the pipe's functionality over its lifetime. The pipe body is generally built up as a combined structure including metallic and polymer layers.

In known flexible pipe design the pipe includes one or more tensile armour layers. The primary load on such a layer is tension. In high pressure applications, the tensile armour layer experiences high tension loads from the internal pressure end cap load as well as weight. This can cause failure in the flexible pipe since such conditions are experienced over prolonged periods of time.

One technique which has been attempted in the past to in some way alleviate the above-mentioned problem is the addition of buoyancy aids at predetermined locations along the length of a riser. Employment of buoyancy aids involves a relatively lower installation cost compared to some other configurations, such as a mid-water arch structure, and also allows a relatively faster installation time. Examples of known riser configurations using buoyancy aids to support the riser's middle section are shown in FIGS. 1 a and 1 b, which show the ‘steep wave’ configuration and the ‘lazy wave’ configuration, respectively. In these configurations, there is provided a riser assembly 200 suitable for transporting production fluid such as oil and/or gas and/or water from a subsea location to a floating facility 202 such as a platform or buoy or ship. The riser is provided as a flexible riser, i.e. including a flexible pipe, and includes discrete buoyancy modules 204 affixed thereto. The positioning of the buoyancy modules and flexible pipe can be arranged to give a steep wave configuration 206 ₁ or a lazy wave configuration 206 ₂.

However, in some applications, the buoyancy modules may react to changes in riser assembly weight, for example caused by marine growth (shellfish and other sea life and/or sea debris attaching to the riser). Alternatively or additionally, the buoyancy modules may experience a gradual or sudden change in content density due to movement or general day to day wear. This may cause the amount of buoyancy support (and therefore the relative height above the sea bed) of the riser to change. Generally, maximum uplift is provided at the beginning of a buoyancy module's life, and uplift decreases over time. Any change in the amount of buoyancy support may have an adverse effect on the tension relief provided to the flexible pipe, which could ultimately decrease the lifetime of a riser.

Furthermore, such changes in weight could lead to an undesirable situation where the riser assembly diverts completely from its designated configuration by either popping up to the water's surface or sinking to the seabed. This is particularly applicable to shallow water applications (less than 1000 feet (304.8 metres)), since any change in buoyancy has a more pronounced effect on the height change at shallow depths. Interference with any neighbouring riser assemblies or vessel structures could become a problem.

In addition, from time to time there are occasions when it would be beneficial to be able to move a section of a riser from its in-use position to another location. However, to move such a piece of apparatus (particularly at any speed) has always proved cumbersome and energy intensive.

It is an aim of the present invention to at least partly mitigate the above-mentioned problems.

It is an aim of embodiments of the present invention to provide a riser system that is more capable of adapting to marine growth, motion from tides and other vessels, and internal fluid density, and a buoyancy element to be used in such a system.

It is an aim of embodiments of the present invention to provide a buoyancy element that can be actively controlled in response to outside variables.

According to a first aspect of the present invention there is provided a buoyancy element for providing buoyancy to a flexible riser, comprising:

-   -   a body of at least partly flexible structure having a volume         that is adjustable between an expanded and contracted position,         and     -   an entry port for fluid to be supplied to and/or removed from         the body to thereby adjust the volume of the body.

According to a second aspect of the present invention there is provided a riser assembly for transporting fluids from a sub-sea location, comprising:

-   -   a flexible pipe; and

one or more buoyancy element, comprising a body of at least partly flexible structure having a volume that is adjustable between an expanded and contracted position, and an entry port for fluid to be supplied to and/or removed from the body to thereby adjust the volume of the body

According to a third aspect of the present invention there is provided a method of supporting a riser, the method comprising the steps of:

-   -   providing a riser comprising at least one segment of flexible         pipe;     -   providing at least one buoyancy element for providing buoyancy         to a portion of the riser, the buoyancy element having a body of         at least partly flexible structure; and     -   adjusting the volume of the body by supplying fluid to and/or         removing fluid from the body.

Certain embodiments of the invention provide the advantage that the volume of fluid within a buoyancy element can be increased and/or decreased enabling the buoyancy level to be changed. Certain embodiments of the invention provide the advantage that a buoyancy element is provided that allows enhanced control over riser configuration. Certain embodiments of the invention enable an operator to have full control of the height of a buoyancy element above the seabed. The buoyancy element can be actively inflated or deflated to adjust its buoyancy in response to external influences on buoyancy such as marine growth, as well as internal influences such as gradual changes in internal fluid density. A riser supported by such buoyancy element would be thereby controlled. The configuration of a riser could be more rapidly changed to avoid passing vessels, or be sunk during a severe storm and later recovered, for example. In certain embodiments of the invention, buoyancy can be continually or intermittently adjusted and controlled as required by an external operator or by internal means.

Embodiments of the invention are further described hereinafter with reference to the accompanying drawings, in which:

FIG. 1 a illustrates a known riser assembly;

FIG. 1 b illustrates another known riser assembly;

FIG. 2 illustrates a flexible pipe body;

FIG. 3 illustrates another riser assembly;

FIG. 4 a illustrates a buoyancy module;

FIG. 4 b illustrates the buoyancy module of FIG. 4 a in use on a flexible pipe;

FIG. 5 illustrates a further buoyancy module in use;

FIG. 6 illustrates a yet further buoyancy module; and

FIG. 7 illustrates a yet further buoyancy module.

In the drawings like reference numerals refer to like parts.

Throughout this description, reference will be made to a flexible pipe. It will be understood that a flexible pipe is an assembly of a portion of a pipe body and one or more end fittings in each of which a respective end of the pipe body is terminated. FIG. 2 illustrates how pipe body 100 is formed in accordance with an embodiment of the present invention from a combination of layered materials that form a pressure-containing conduit. Although a number of particular layers are illustrated in FIG. 2, it is to be understood that the present invention is broadly applicable to coaxial pipe body structures including two or more layers manufactured from a variety of possible materials. It is to be further noted that the layer thicknesses are shown for illustrative purposes only.

As illustrated in FIG. 2, a pipe body includes an optional innermost carcass layer 101. The carcass provides an interlocked construction that can be used as the innermost layer to prevent, totally or partially, collapse of an internal pressure sheath 102 due to pipe decompression, external pressure, and tensile armour pressure and mechanical crushing loads. It will be appreciated that certain embodiments of the present invention are applicable to ‘smooth bore’ operations (i.e. without a carcass) as well as such ‘rough bore’ applications (with a carcass).

The internal pressure sheath 102 acts as a fluid retaining layer and comprises a polymer layer that ensures internal fluid integrity. It is to be understood that this layer may itself comprise a number of sub-layers. It will be appreciated that when the optional carcass layer is utilised the internal pressure sheath is often referred to by those skilled in the art as a barrier layer. In operation without such a carcass (so-called smooth bore operation) the internal pressure sheath may be referred to as a liner.

An optional pressure armour layer 103 is a structural layer with a lay angle close to 90° that increases the resistance of the flexible pipe to internal and external pressure and mechanical crushing loads. The layer also structurally supports the internal pressure sheath, and typically consists of an interlocked construction.

The flexible pipe body also includes an optional first tensile armour layer 105 and optional second tensile armour layer 106. Each tensile armour layer is a structural layer with a lay angle typically between 10° and 55°. Each layer is used to sustain tensile loads and internal pressure. The tensile armour layers are often counter-wound in pairs.

The flexible pipe body shown also includes optional layers of tape 104 which help contain underlying layers and to some extent prevent abrasion between adjacent layers.

The flexible pipe body also typically includes optional layers of insulation 107 and an outer sheath 108, which comprises a polymer layer used to protect the pipe against penetration of seawater and other external environments, corrosion, abrasion and mechanical damage.

Each flexible pipe comprises at least one portion, sometimes referred to as a segment or section of pipe body 100 together with an end fitting located at at least one end of the flexible pipe. An end fitting provides a mechanical device which forms the transition between the flexible pipe body and a connector. The different pipe layers as shown, for example, in FIG. 2 are terminated in the end fitting in such a way as to transfer the load between the flexible pipe and the connector.

FIG. 3 illustrates a riser assembly 300 suitable for transporting production fluid such as oil and/or gas and/or water from a sub-sea location 301 to a floating facility 302. For example, in FIG. 3 the sub-sea location 301 includes a sub-sea flow line. The flexible flow line 305 comprises a flexible pipe, wholly or in part, resting on the sea floor 304 or buried below the sea floor and used in a static application. The floating facility may be provided by a platform and/or buoy or, as illustrated in FIG. 3, a ship. The riser assembly 300 is provided as a flexible riser, that is to say a flexible pipe 303 connecting the ship to the sea floor installation. The flexible pipe may be in segments of flexible pipe body with connecting end fittings.

It will be appreciated that there are different types of riser, as is well-known by those skilled in the art. Embodiments of the present invention may be used with any type of riser, such as a freely suspended (free, catenary riser), a riser restrained to some extent (buoys, chains), totally restrained riser or enclosed in a tube (I or J tubes).

FIG. 3 also illustrates how portions of flexible pipe can be utilised as a flow line 305 or jumper 306.

An example of a buoyancy module 400 according to the present invention is illustrated in FIG. 4 a. In this embodiment, the buoyancy module is an inflatable bladder having a flexible body 402 of heavy duty plastic material. The buoyancy module also has a port 404 formed by an opening in the body 402. The port 404 acts as a fluid supply/removal means for fluid to be introduced and removed from the body 402.

The buoyancy module is shown in use in FIG. 4 b, in which the buoyancy module 400 is provided directly around a flexible pipe 450 which itself forms a riser.

In more detail, a conduit, here a fluid supply tube 406, is connected to the port 404 and runs along the outside of the flexible pipe 450 to link an external fluid source 408, which in this case is an air flow system located on a vessel carrying the riser, to the port 404.

The air flow system 408 may include a pump system for moving air down the tube 406, towards and into the buoyancy module 400, and a suction system for sucking air from the buoyancy module 400 up the tube 406 and away for removal. The port 404 is aptly fitted with a suitable two-way valve for maintaining the fluid within the body 402 until overcome by a predetermined air flow pressure in either direction.

It will be appreciated that the tube 406 need not function as both the delivery and removal tube. For example, the tube 406 could function only as a fluid delivery tube. A fluid removal system may not be required, or a separate outlet port could be provided for dispelling fluid from the body 402 to the exterior (either via a further tube to the surface or simply directly into the surrounding sea). It will also be appreciated that the fluid need not be air. The fluid could be many types of gas or gas mixtures, having a weight/density less than water (per unit volume). The fluid could also be a liquid.

Aptly, the buoyancy module could include a monitoring device such as a depth gauge for monitoring the relative buoyancy of the buoyancy module. The depth gauge could be linked to a controller system at the surface, by wire or other signaling system. The controller system could be manually operated at the surface. Then, the buoyancy of the buoyancy module and adjacent riser portion can be monitored, and the operator can operate the pump system and suction system as appropriate to alter the volume of air in the body, and thus alter the buoyancy of the buoyancy module. The monitoring device could alternatively be a fluid density monitor or other suitable device.

With the above-described buoyancy module 400, the volume of fluid within the module can be increased or decreased enabling the buoyancy level to be increased or decreased.

In a further embodiment of the present invention, a buoyancy module 500 is provided with both a gas and liquid supply (see FIG. 5). The principle of changing the fluid volume within the body 502 may be used to affect the buoyancy level. However, when needed, the addition of a liquid into the buoyancy module (of a relatively higher density than the gas) would decrease the buoyancy, and possibly compress the gas element present. Thus the availability of two different elements for affecting the buoyancy level gives a further degree of freedom to the system. In this embodiment the gas and liquid are supplied through separate hoses 506, 508. The gas supply tube 506 is connected to the port 504, and liquid supply tube 508 is connected to a port 510, and each tube runs along the outside of the flexible pipe 550 to each link to an external fluid source. Alternatively the gas and liquid could be supplied through a single hose. There may be a single outlet hose or port, or separate outlet hoses/ports.

The buoyancy module 500 includes a depth gauge 512 for monitoring the relative buoyancy of the buoyancy module. The depth gauge 512 is linked to a controller system 514 at the surface, by wire or other signaling system. The controller system in this example is manually operated at the surface. The buoyancy of the buoyancy module and adjacent riser portion is monitored, and the operator can operate a pump and suction system as appropriate (for example similarly to the air flow system 408 described above) to alter the volume of fluids in the body 502, and thus alter the buoyancy of the buoyancy module 500. The monitoring device 512 could alternatively be a fluid density monitor or other suitable device.

With the above-described buoyancy module, the buoyancy level can be actively controlled, periodically or constantly throughout the entire lifetime of buoyancy module and the flexible pipe to which it may be attached. The fluid content of the buoyancy module can be actively changed via the supply port (and/or exit port) to increase or decrease buoyancy as required. The at least partly flexible body portion of the buoyancy module helps in the adjustment of fluid content, since the volume within the buoyancy module is not fixed. This adjustment of buoyancy may be occasional or periodical injections of fluid to counteract gradual changes in buoyancy, such as that caused by marine growth over buoyancy modules and flexible pipes. The adjustment in buoyancy may be a large, sudden change in buoyancy, to rapidly change the position of the riser, to avoid a passing vessel for example. The riser could be sunk to a deeper level below the sea surface during a storm, and then raised back upwards to its original position when it is safe to do so. The height in water depth of a flexible pipe can be changed. The buoyancy may be constantly monitored throughout the lifetime of a riser assembly. The controller may be automatic, such as a computer system arranged to receive signals relating to the buoyancy and instruct injection or release of fluid. The controller may be a human, manually analysing the buoyancy level and changing the fluid volume as necessary. The controller could be a combination of both automatic and manual operation.

FIG. 6 shows a further embodiment of the present invention. In this embodiment a buoyancy module 600 includes a flexible, conformable bladder 602 and a rigid support structure 604. The support structure may be formed of steel, for example. The support structure 604 includes clamps 606 for clamping the buoyancy module 600 to a flexible pipe 650. A fluid supply tube 608 is connected via the support structure 604 to the bladder 602, and at its distal end (not shown) to a fluid source. The clamps allow the buoyancy module to be securely attached to a flexible pipe. The clamps also allow the inflatable portion of the buoyancy module to not necessarily directly surround the flexible pipe. That is, in this embodiment, the bladder 602 is offset to one side of the flexible pipe 650.

FIG. 7 shows a yet further embodiment of the present invention. In this embodiment a buoyancy module 700 includes a body portion including a flexible bladder 702 and a rigid outer shield 704. The bladder 702 is positioned directly around the flexible pipe 750 in use. The outer shield 704 is positioned to partly cover the bladder 702, acting as a protective armouring to protect the bladder 702 from abrasion, puncture or other damage at least from one direction. For example, the shield 704 may be provided on the upper side of the buoyancy module 700 so as to give resistance and protection in the event that a passing vessel was to strike the buoyancy module. The buoyancy module 700 also includes a filler hose 706 connecting a fluid source (not shown) to the bladder 702 via the shield 704, for allowing fluid to be introduced into the bladder 702.

With the above-described buoyancy modules, the volume of fluid within the module can be increased and/or decreased enabling the buoyancy level to be changed. In certain embodiments the volume and/or density of fluid within the module can be increased or decreased. In certain embodiments the buoyancy level can be monitored and adjusted in response thereto.

The present invention as described herein may offer a reduced cost and/or reduced installation time compared to other fixed buoyancy support structures such as a mid-water arch or other more complex support structures including buoyancy modules.

Various modifications to the detailed designs as described above are possible. For example, more than one buoyancy module as described above can be used support a flexible pipe as needed. Generally the weight of a riser for the oil and gas industry may require more than one buoyancy module, depending on the size of the riser. In this case, the plurality of buoyancy modules may be each linked in series or in parallel to a fluid supply/removal system. With additional controllable buoyancy modules, the configuration of the riser would become more controllable and the overall shape of the riser can be precisely arranged and manoeuvred in response to external influences.

Although the examples described above include a fluid supply tube external to the flexible pipe, a tube for performing the same function could instead be incorporated into the flexible pipe within a layer of the pipe body such as the outer sheath layer. The connection of the tube to the buoyancy module would be adapted accordingly. Of course, the fluid supply tube need be only a small diameter hose in comparison to the diameter of a flexible pipe. As such, the small diameter hose will have a high resistance to collapse under water, and need not be protected from external pressure by complex structural arrangements.

With any of the above-described embodiments, the buoyancy module may further optionally include a check valve configured to allow a predetermined maximum pressure of fluid to be present in the body of the buoyancy module. The check valve could be installed on the fluid supply line, on the buoyancy module itself, or at the host vessel. Such a feature would ensure that the buoyancy module could not be accidentally over-inflated, for example. In addition, in the event that a riser were to rapidly ascend, fluid volume within the buoyancy module would rapidly expand. A suitable valve could also be used to relieve the pressure by allowing fluid to be exhausted. Alternatively or additionally, an alarm means could be provided for warning against over inflation or under inflation.

With any of the above-described embodiments, a riser assembly may include further optional features so as to prevent rapid pressurization or depressurization. For example, to prevent rapid ascent of the riser assembly due to over-pressurization, the riser assembly may include a tether arrangement such as a chain connecting the riser to an anchor on the seabed or a platform. The tether may be fixed around a suitable connecting portion of the riser, and of a length so as to restrict the movement of the riser beyond a certain predetermined limit.

A riser assembly may optionally include wing-type sections for causing a certain amount of drag on the riser when the riser moves through water. The wing-type sections of the riser would decrease the speed at which the riser assembly may travel through water, so as to prevent very a rapid descent of the riser through water (which could lead to damage of the riser upon hitting the seabed or other surface).

Alternatively or additionally, a riser system may include one or more known ‘standard’ buoyancy modules, of substantially fixed buoyancy and not connected to a fluid supply/removal system. This would help ensure that the riser assembly only ever descended to a predetermined depth below the surface. Again this would help prevent damage to the riser in the case of depressurization.

It will be clear to a person skilled in the art that features described in relation to any of the embodiments described above can be applicable interchangeably between the different embodiments. The embodiments described above are examples to illustrate various features of the invention.

Throughout the description and claims of this specification, the words “comprise” and “contain” and variations of them mean “including but not limited to”, and they are not intended to (and do not) exclude other moieties, additives, components, integers or steps. Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

Features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. The invention is not restricted to the details of any foregoing embodiments. The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

The reader's attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference. 

1. A buoyancy element for providing buoyancy to a flexible riser, comprising: a body of at least partly flexible structure having a volume that is adjustable between an expanded and contracted position, and an entry port for fluid to be supplied to and/or removed from the body to thereby adjust the volume of the body in response to a signal from a control means external to the buoyancy element, and a depth monitoring device for communicating depth or buoyancy level to the control means.
 2. A buoyancy element as claimed in claim 1, further comprising a conduit in fluid communication with the entry port for supplying fluid to and/or removing fluid from the body.
 3. A buoyancy element as claimed in claim 1, wherein the entry port is configured for fluid to be supplied to and removed from the body.
 4. A buoyancy element as claimed in claim 2 further comprising a further tube in fluid communication with a further entry port to the body.
 5. A buoyancy element as claimed in claim 1 further comprising one or more check valve configured to allow a maximum pressure of fluid within the body.
 6. A buoyancy element as claimed in claim 1 wherein the body is an inflatable bladder.
 7. A buoyancy element as claimed in claim 1, wherein the body comprises a rigid support structure portion and a flexible portion.
 8. A buoyancy element as claimed in claim 1, further comprising a clamping element for clamping the buoyancy element to a flexible riser.
 9. A buoyancy element as claimed in claim 8, wherein the clamping element is attached to the rigid support structure portion of the body.
 10. (canceled)
 11. (canceled)
 12. A riser assembly for transporting fluids from a sub-sea location, comprising: a flexible pipe; and one or more buoyancy element, comprising a body of at least partly flexible structure having a volume that is adjustable between an expanded and contracted position, an entry port for fluid to be supplied to and/or removed from the body to thereby adjust the volume of the body in response to a signal from a control means external to the buoyancy element, and a depth monitoring device for communicating depth or buoyancy level to the control means.
 13. A riser assembly as claimed in claim 12, further comprising a tethering element for tethering the riser assembly to a fixed structure.
 14. A riser assembly as claimed in claim 12, further comprising drag elements for decreasing the speed at which the flexible riser can move underwater.
 15. A riser assembly as claimed in claim 12, further comprising at least one buoyancy element of a substantially fixed buoyancy, not connected to a fluid supply/removal conduit.
 16. A method of supporting a riser, the method comprising the steps of: providing a riser comprising at least one segment of flexible pipe; providing at least one buoyancy element for providing buoyancy to a portion of the riser, the buoyancy element having a body of at least partly flexible structure; providing a control means external to the buoyancy element; providing a depth monitoring device for communicating depth or buoyancy level to the control means; and adjusting the volume of the body by supplying fluid to and/or removing fluid from the body in response to a signal from the control means. 17-19. (canceled) 